The present invention is directed to the field of digital signal processing circuits and more particularly to electronic ignition timing systems which use such circuits.
Generally, digital circuits produce an output count which is a non-linear function of an input signal by first converting the input signal, if it is in analog form, to a digital input signal and then using a read only memory (ROM) which receives the digital input signal and produces a corresponding output signal that is generally coupled to a holding register which stores the output of the ROM. The non-linear relationship between the digital input signal to the ROM and the count stored in the holding register is obtained by having the ROM effectively perform a point by point plot, or table look up, in order to produce the desired relationship between the digital input signal and the output count stored in the holding register. This point by point plotting requires an extremely large memory capacity for the ROM in order to accurately produce the output count which has the desired non-linear relationship to the digital input signal. By requiring a large ROM memory, the cost of these type of prior art digital processing circuits has been relatively large and has thus discouraged their use in commercial products, such as electronic ignition timing systems for internal combustion engines.
Generally, electronic ignition timing systems have used complex and costly analog circuits in order to generate output functions which have a non-linear relationship to a sensed variable engine condition. Such circuits generally employ a plurality of zener diodes which break down at various voltages in order to produce a piecewise linear (and therefore non-linear) output function for a variable magnitude input signal related to a variable engine condition. The reason that such circuits have been used in the past is that it is well known that the desired spark timing advance characteristic verses engine speed has a piecewise linear shape over the entire range of anticipated engine speeds. In addition, the desired advance characteristic as a function of other engine variables has also been found to have a piecewise linear shape as a function of the magnitude of these other variables.
The prior art electronic ignition timing systems have used either complex and costly analog circuitry to produce output functions having a piecewise linear relationship to variable engine conditions, or they have used large capacity memory ROMs to produce point by point plotting of a non-linear output function as a function of a digital input signal. An example of such an analog ignition system is shown in a U.S. Pat. No. to Niemoeller, 3,785,356, and an example of such a digital electronic ignition system is shown in a U.S. Pat. No. to Asplund, 3,749,073.